Yeah, yeah, I know this might sound crazy but just look for a second. I have heard people say that Mata-Nui and Makuta are opposites, you know like yin and yang, life and death, and most importantly Light and Darkness/shadow. But wait, isn't Takanuva Light and Makuta darkness/shadow? Why, yes they are. And that is one reason why Takanuva and Mata-Nui are the same. Also in MOL Takutanuva says that Mata-Nui will be awakend and then he dies along with Takanuva, after that happens they take the MoL lay it on the unity, duty, destiny sign and Takanuva was awakend from (as Yoda would say) Forever SLEEP just as Mata-Nui would have been. That is reason number 2. And finally reason number 3. In MOL at kini-nui after they beat the rahkshi the mask there becomes a pakari, Takanuva's old mask, and I think that the mask at Kini-Nui represents Mata-Nui's mask.
Well, there you have it. That is what I think. Post your thoughts about my theory.